RNA Splicing: Removal of Introns from Primary ... - EURASNET

AT-AC introns IIOf note is that introns with GT-AG borders, but which are spliced by the U12 spliceosome,and introns with AT-AC borders, spliced by the classical U2 spliceosome also occur,at a frequency comparable to that of the U12-type with AT-AC terminiHence, residues other than terminal dinucleotides determine which of the two spliceosomeswill be utilisedU12 class introns represent approximately 0.1% of all intronsThey are found in organisms ranging from higher plants to mammals,and their positions within equivalent genes are frequently phylogenetically conservedThe genomes of Saccharomyces cerevisiae and Caenorhabditis elegans contain no U12-type intronsSince U12 introns clearly originated prior to the divergence of the plant and animal kingdoms, their absencein C. elegans is most easily explained by their conversion to U2-type introns or by intron loss,rather than by intron gain in plants and vertebratesAGU6atac (At/Hs)AGGAAAUAUCCUUYUUCGGGAAAAAU11 (Hs)U12 (At/Hs)AGGAAU-GUCCUUAAC10-16 ntYYCACG